NAL-NL2 Empirical Adjustments

Abstract

NAL-NL1, the first procedure from the National Acoustic Laboratories (NAL) for prescribing nonlinear gain, was a purely theoretically derived formula aimed at maximizing speech intelligibility for any input level of speech while keeping the overall loudness of speech at or below normal loudness. The formula was obtained through an optimization process in which speech intelligibility and loudness were predicted from selected models. Using updated models and applying some revisions to the derivation process, a theoretically derived NAL-NL2 formula was obtained in a similar way. Further adjustments, directed by empirical data collected in studies using NAL-NL1 as the baseline response, have been made to the theoretically derived formula. Specifically, empirical data have demonstrated that (a) female hearing aid users prefer lower overall gain than male users; (b) new hearing aid users with more than a mild hearing loss prefer increasingly less gain with increasing degree of hearing loss than experienced hearing aid users, and require up to 2 years to adapt to gain levels selected by experienced hearing aid users; (c) unilaterally and bilaterally fitted hearing aid users prefer overall gain levels that vary less than estimated by the bilateral correction factor; (d) adults prefer lower overall gain than children; and (e) people with severe/profound hearing loss prefer lower compression ratios than predicted when fitted with fast-acting compression. The literature and data leading to these conclusions are summarized and discussed in this article, and the procedure for implementing the adjustments to the theoretically derived NAL-NL2 formula is described.

Effects of Removing Low-Frequency Electric Information on Speech Perception With Bimodal Hearing

“…However, newer prescriptions have been developed in recent years. For instance, the National Acoustic Laboratories Nonlinear 2 strategy takes into account additional variables for prescription of gain, including previous HA experience, gender preferences, and severe-to-profound HL (Keidser, Dillon, Carter, & O'Brien, 2012). The NAL-Revised, Profound fitting formula is another strategy developed specifically for individuals with severe-to-profound HL, which prescribes less low-frequency gain for flat and rising HL configurations and more high-frequency amplification for steeply sloping HL (Byrne, Dillon, Ching, Katsch, & Keidser, 2001).…”

Hearing Aid Use in Older Adults With Postlingual Sensorineural Hearing Loss: Protocol for a Prospective Cohort Study (Preprint)

“…The hearing aids will be fitted by the participating audiology clinics according to the best-practice fitting guidelines including real ear (probe tube) measures to verify the amplification and match to appropriate prescribed amplification (typically using the NAL-NL2 prescription developed by the National Acoustics Laboratory [43]), with further adjustment and fine-tuning based on the users' subjective preferences. The study audiologists will provide oral explanation on how to use hearing aids and a step-by-step guide on hearing aid use will be provided to participants as take home materials.…”

Self-Fitting Algorithm for Digital Hearing Aid Based on Interactive Evolutionary Computation and Expert System

“…However, individual differences such as age, sex or service life of the hearing aid affect the fitting results, and thus the average approach is biased [22]. Therefore, how to effectively adjust the parameters is quite difficult [23,24].…”

“…Even with the same audiogram, the real fitting results are influenced by the patient's cognitive level and personal habits, among other factors [22]. So, the proposed self-fitting algorithm optimally sets the signal gain set according to the patient information [25] and feedback.…”